The studies described in this research proposal include: (1) the chemical nature of selenium in the selenoprotein, glutathione peroxidase, (2) pathways for the metabolism of sodium selenite via glutathion derivatives or other intermediates to biologically active forms of selenium as well as urinary metabolites and detoxified forms, (3) the mechanism by which Cd selectively damages the testes vascular system, (4) the mechanisms by which selenium prevents the toxic effects of certain heavy metals, such as silver, cadmium, and mercury, and (5) the role of selenium in animals' mechanisms for resistance to infection.